Labeled compounds for measuring the function of the muscarinic acetylcholine nervous system

ABSTRACT

A muscarinic acetylcholine nervous system labeled compound for positron emission tomography measurement of the present invention has a structure represented by undermentioned general formula (I):  
                 
 
     [in the formula, W represents one selected from the group consisting of groups represented by undermentioned formulae (II) and (III):  
                 
 
     (in the formulae, R represents one selected from the group consisting of an  11 C-labeled ethyl group and an  11 C-labeled propyl group), and in the case that W is a group represented by above-mentioned formula (II), above-mentioned formula (I) is the (+)-isomer].

TECHNICAL FIELD

[0001] The present invention relates to a muscarinic acetylcholinenervous system labeled compound, and more specifically to a muscarinicacetylcholine nervous system labeled compound for positron emissiontomography (PET) measurement.

BACKGROUND ART

[0002] With an aging society, the increasing economic burden due to anincrease in the number of people with dementia is becoming a socialproblem, and hence elucidation of the condition of dementia anddevelopment of effective anti-dementia drugs are needed.

[0003] The pathology of dementia is not yet clear, but research hithertohas suggested that deterioration of the functioning of the acetylcholinenervous system is one cause of dementia. Evaluation of the functioningof the acetylcholine nervous system thus plays a very important role inelucidating the condition of dementia and in developing anti-dementiadrugs, and hence there are strong calls for the development ofnon-intrusive methods of carrying out such evaluation and of labeledcompounds suitable for use in these methods.

[0004] A method in which positron emission tomography (hereinafterreferred to as ‘PET’) measurements are carried out using a labeledcompound possessing a positron-emitting nuclear species is known as anon-intrusive method for evaluating the functioning of the acetylcholinenervous system. In this method, acetylcholine nervous system receptorsare labeled using the labeled compound, and γ-rays that are emitted uponpositrons emitted from the emitting nuclear species combining withmatter-constituting electrons and annihilation taking place aremeasured, thus measuring the distribution of the receptors.

[0005] [¹¹C] N-methyl-4-piperidyl benzilate (hereinafter referred to as‘[¹¹C]4-NMPB’) has been proposed as a muscarinic acetylcholine nervoussystem labeled compound for use in such PET measurements. However, therehave been problems such as the following with PET measurements using[¹¹C]4-NMPB:

[0006] (i) the ionicity of [¹¹C]4-NMPB is low in the blood, and hencethe liposolubility is high, and thus the ability to migrate into tissueis good, but the amount of [¹¹C]4-NMPB that binds non-specifically tomuscarinic acetylcholine nervous system receptors in the tissue is high,and hence errors are prone to arising when measuring the amount of[¹¹C]4-NMPB that binds specifically to the receptors;

[0007] (ii) [¹¹C]4-NMPB has a structure for which optical isomers do notexist, and hence errors are prone to arising when measuring the amountof [¹¹C]4-NMPB that binds specifically to muscarinic acetylcholinenervous system receptors;

[0008] (iii) the affinity of [¹¹C]4-NMPB to the receptors is high andthe dissociation constant (k₄) is very low, and hence the movement of[¹¹C]4-NMPB in the brain is prone to being affected by changes in thelocal blood flow amount, and thus in the case of a disease model or acondition accompanied by a drop in cerebral circulation, it is difficultto distinguish whether measurement results are due to genuine changes inthe activity of the muscarinic acetylcholine nervous system receptors ormerely due to changes in the local blood flow amount in the brain;

[0009] (iv) the affinity of [¹¹C]4-NMPB to the muscarinic acetylcholinenervous system receptors is very high compared with that ofacetylcholine, which is an intrinsic neurotransmitter, and hence it isdifficult to measure competition on the receptors between acetylcholinethat has been discharged from preganglionic nerves due toneurotransmission and the labeled compound.

[0010] Moving on, it is disclosed in Japanese Patent ApplicationLaid-open No. 11-152270 that by using [¹¹C](±)N-methyl-3-piperidylbenzilate (hereinafter referred to as ‘[¹¹C](±)3-NMPB’), PETmeasurements can be carried out with higher precision than when[¹¹C]4-NMPB is used. However, even using [¹¹C](±)3-NMPB has still notbeen sufficient with regard to carrying out PET measurements withimproved precision.

DISCLOSURE OF THE INVENTION

[0011] In view of the problems of the prior art described above, it isan object of the present invention to provide a muscarinic acetylcholinenervous system labeled compound that enables PET measurements to becarried out efficiently with improved precision and without there beinginfluence from changes in the blood flow amount in the region ofinterest in the brain, a method of manufacturing the labeled compound,and a positron emission tomography measurement method using the labeledcompound.

[0012] The present inventors carried out assiduous studies to attain theabove object, and as a result discovered that the problems describedabove can be resolved by using an ¹¹C-labeled benzilic acidalkylpiperidyl ester having a specific structure as a muscarinicacetylcholine nervous system labeled compound, thus arriving at thepresent invention.

[0013] Specifically, a muscarinic acetylcholine nervous system labeledcompound for positron emission tomography measurement of the presentinvention is characterized by having a structure represented byundermentioned general formula (I):

[0014] [in the formula, W represents one selected from the groupconsisting of groups represented by undermentioned formulae (II) and(III):

[0015] (in the formulae, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup), and in the case that W is a group represented by above-mentionedformula (II), above-mentioned formula (I) is the (+)-isomer].

[0016] Moreover, a method of manufacturing a muscarinic acetylcholinenervous system labeled compound for positron emission tomographymeasurement of the present invention is characterized by comprising astep of obtaining a compound represented by undermentioned formula (I):

[0017] from an ¹¹C-labeled alkyl halide represented by undermentionedformula (IV):

R—X   (IV)

[0018] and a benzilic acid piperidyl ester represented by undermentionedformula (V):

[0019] [In the formulae, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup, X represents a halogen atom, W′ represents one selected from thegroup consisting of a (+) 3-piperidyl group and a 4-piperidyl group, andW represents one selected from the group consisting of groupsrepresented by undermentioned formulae (II) and (III):

[0020] (in the formulae, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup).]

[0021] Furthermore, a positron emission tomography measurement method ofthe present invention comprises: a step of administering theabove-mentioned labeled compound of the present invention to a subject,thus labeling muscarinic acetylcholine nervous system receptors of thesubject with the labeled compound; and a step of measuring γ-raysemitted through the combination of positrons emitted from an emittingnuclear species possessed by the labeled compound and prescribedmatter-constituting electrons.

[0022] According to the present invention, by using a labeled compoundrepresented by above-mentioned formula (I), in PET measurements, theamount of γ-rays corresponding to specific binding between the labeledcompound and the above-mentioned receptors can be measured with highprecision without there being influence from changes in the blood flowamount. It thus becomes possible to obtain information on theacetylcholine nervous system in prescribed regions of interest in thebrain with high precision. Moreover, the affinity of the labeledcompound of the present invention to the receptors is lower than that ofconventional labeled compounds, and binding of the labeled compound tothe receptors and dissociation of the labeled compound from thereceptors occurs in a relatively short time, and hence PET measurementscan be carried out efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIGS. 1A to 1D are explanatory drawings showing the structures ofmuscarinic acetylcholine nervous system labeled compounds of the presentinvention.

[0024]FIGS. 2A to 2E are graphs showing HPLC analysis results for¹¹C-labeled alkyl iodides obtained in Examples 1 to 4 and ComparativeExamples 1 and 2.

[0025]FIGS. 3A to 3C are graphs showing HPLC analysis results forlabeled compounds obtained in Examples 1 and 2 and Comparative Example1; in each graph the full line shows results of analysis in radioactivemode, and the broken line shows results of analysis in UV mode(wavelength 235 nm).

[0026]FIGS. 4A to 4C are graphs showing HPLC analysis results forlabeled compounds obtained in Examples 3 and 4 and Comparative Example2; in each graph the full line shows results of analysis in radioactivemode, and the broken line shows results of analysis in UV mode(wavelength 235 nm).

[0027]FIGS. 5A to 5C are graphs showing the relationship betweenmeasurement time and radioactivity concentration in regions of interest,obtained from PET measurements using [¹¹C](+)3-NMPB, [¹¹C](+)3-NEPB and[¹¹C](+)3-NPPB.

[0028]FIGS. 6A to 6C are graphs showing the relationship betweenmeasurement time and radioactivity concentration in regions of interest,obtained from PET measurements using [¹¹C]4-NMPB, [¹¹C]4-NEPB and[¹¹C]4-NPPB.

[0029]FIGS. 7A to 7C are graphs showing the relationship betweenmeasurement time and radioactivity concentration in regions of interest,obtained from PET measurements using [¹¹C](+)3-NPPB, for the cases ofadministering 0 μg/kg (no administration), 50 μg/kg and 250 μg/kgrespectively of E2020 to a subject.

[0030]FIGS. 8A to 8C are graphs showing the relationship betweenmeasurement time and radioactivity concentration in regions of interest,obtained from PET measurements using [¹¹C]4-NMPB, for the cases ofadministering 0 μg/kg (no administration), 50 μg/kg and 250 μg/kgrespectively of E2020 to a subject.

BEST MODES FOR CARRYING OUT THE INVENTION

[0031] Following is a detailed description of preferable embodiments ofthe present invention, with reference to the drawings in places.

[0032] A labeled compound of the present invention has a structurerepresented by undermentioned general formula (I):

[0033] [in the formula, W represents one selected from the groupconsisting of groups represented by undermentioned formulae (II) and(III):

[0034] (in the formulae, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup), and in the case that W is a group represented by above-mentionedformula (II), above-mentioned formula (I) is the (+)-isomer]; morespecifically, labeled compounds of the present invention are:

[0035] [¹¹C](+)-N-ethyl-3-piperidyl benzilate (hereinafter referred toas ‘[¹¹C](+)3-NEPB’) as shown in FIG. 1A;

[0036] [¹¹C](+)N-propyl-3-piperidyl benzilate (hereinafter referred toas ‘[¹¹C](+)3-NPPB’) as shown in FIG. 1B;

[0037] [¹¹C]N-ethyl-4-piperidyl benzilate (hereinafter referred to as‘[¹¹C]4-NEPB’) as shown in FIG. 1C; and

[0038] [¹¹C]N-propyl-4-piperidyl benzilate (hereinafter referred to as‘[¹¹C]4-NPPB’) as shown in FIG. 1D.

[0039] Out of the groups represented by R in above-mentioned formulae(II) and (III) , the ¹¹C-labeled propyl group includes both an¹¹C-labeled n-propyl group and an ¹¹C-labeled isopropyl group;nevertheless, an ¹¹C-labeled n-propyl group is preferable, since in thiscase it tends to be that the target labeled compound can be obtainedeasily and reliably.

[0040] Moreover, the optical isomers [¹¹C](−)3-NEPB and [¹¹C](−)3-NPPBare present in [¹¹C](+)3-NEPB and [¹¹C](+)3-NPPB respectively; the(+)-isomers, which are the labeled compounds of the present invention,may be used in the form of a mixture with the respective (−)-isomer,i.e. in the form of [¹¹C](±)3-NEPB or [¹¹C](±)3-NPPB, or may be usedafter separating the (+)-isomer from the mixture. An example of a methodof isolating the (+)-isomer from the mixture of optical isomers isoptically resolving column chromatography using a column in which apolysaccharide derivative is supported on silica gel.

[0041] Moreover, [¹¹C](−)3-NEPB and [¹¹C](−)3-NPPB are not able to bindspecifically to the above-mentioned receptors, and hence, for example,by carrying out PET measurements using [¹¹C](+)3-NEPB or [¹¹C](+)3-NPPBalone, and comparing with measurement results for the case that[¹¹C](−)3-NEPB or [¹¹C](−)3-NPPB is used alone, it is possible todetermine the amount of the labeled compound that is bound specificallyto the receptors with improved precision.

[0042] Next, a description will be given of a method of manufacturingthe compounds (I) of the present invention.

[0043] A compound (I) of the present invention can be obtained by themethod represented by undermentioned reaction formula (A):

[0044] [in the formula, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup, X represents a halogen atom, W′ represents one selected from thegroup consisting of a (+)3-piperidyl group and a 4-piperidyl group, andW represents one selected from the group consisting of groupsrepresented by undermentioned formulae (II) and (III):

[0045] (in the formulae, R represents one selected from the groupconsisting of an ¹¹C-labeled ethyl group and an ¹¹C-labeled propylgroup), wherein in the case that W is a group represented byabove-mentioned formula (II), above-mentioned formula (I) is the(+)-isomer, and in the case that W′ is a (+)3-piperidyl group, W is agroup represented by above-mentioned formula (II), whereas in the casethat W′ is a 4-piperidyl group, W is a group represented byabove-mentioned formula (III)]; i.e. by reacting an ¹¹C-labeled alkylhalide (IV) and a benzilic acid piperidyl ester (V) together. Here,examples of X (the halogen atom) in above-mentioned formula (IV) includea chlorine atom, a bromine atom and an iodine atom, with an iodine atombeing preferable. If the halogen atom of the ¹¹C-labeled alkyl halide(IV) is an iodine atom, then it tends to be possible to carry out thereaction easily and efficiently. Moreover, there are no particularlimitations on the method or reaction conditions of the above reaction,with it being possible to use ones that have been publicly known fromhitherto. For example, the target compound (I) can be obtained bycarrying out the above reaction in a dimethylformamide (DMF) solventwith a reaction time of 3 to 6 minutes and a reaction temperature of100° C. Furthermore, in the reaction, dimethylsulfoxide (DMSO) and so oncan be used as the solvent instead of dimethylformamide (DMF). Theidentity of the compound (I) obtained through the reaction can beverified using column chromatography, nuclear magnetic resonance (NMR)absorption spectroscopy using an optically isomeric shift reagent,optical rotation angle measurement and so on.

[0046] There are no particular limitations on the method of synthesizingthe ¹¹C-labeled alkyl halide (IV) that is a starting material of thereaction represented by above-mentioned reaction formula (A); forexample, an ¹¹C-labeled alkyl iodide (IV-a) can be obtained by themethod represented by undermentioned reaction formula (B):

[0047] (in the formula, R′ represents one selected from the groupconsisting of a methyl group and an ethyl group); i.e. by reacting¹¹C-labeled carbon dioxide (VI) and an alkyl magnesium bromide (VII)together, reducing the compound (VIII) thus obtained using a reducingagent such as LiAlH₄, and then treating with hydroiodic acid. In theabove reaction, commercially sold ¹¹C-labeled carbon dioxide may be usedas the ¹¹C-labeled carbon dioxide (VI), or ¹¹C-labeled carbon dioxidesynthesized using a method such as a ¹⁴N(p, α) ¹¹C reaction may be used.Furthermore, regarding the alkyl magnesium bromide (VII) as well, eithera commercially sold one or one synthesized using a method that has beenpublicly known since hitherto may be used.

[0048] Moreover, there are no particular limitations on the method ofsynthesizing the benzilic acid piperidyl ester (V) that is a startingmaterial of the reaction represented by above-mentioned reaction formula(A); for example, the benzilic acid piperidyl ester (V) can be obtainedby the method represented by undermentioned reaction formula (C):

[0049] (in the formula, R″ represents an alkyl group, and W′ representsone selected from the group consisting of a 3-piperidyl group and a4-piperidyl group); i.e. by reacting a benzilic acid ester (IX) and apiperidinol (X) together. Here, examples of the benzilic acid ester (IX)include methyl benzilate, ethyl benzilate, propyl benzilate and butylbenzilate. Moreover, in the case that the piperidinol (X) is3-piperidinol, optical isomers exist; (+)3-piperidinol that has beenisolated in advance by optical resolution may be used, or a mixture ofthe optical isomers (±)3-piperidinol may be used. In the case that(±)3-piperidinol is used, the (+)-isomer can be obtained by carrying outoptical resolution using optically resolving column chromatography orthe like after above-mentioned reaction (A) or (C).

[0050] By using a labeled compound of the present invention obtained inthis way, in PET measurements, the amount of γ-rays corresponding tospecific binding between the labeled compound and the above-mentionedreceptors can be measured with high precision without there beinginfluence from changes in the blood flow amount. It thus becomespossible to obtain information on the acetylcholine nervous system inprescribed regions of interest in the brain with high precision.Moreover, the affinity of the labeled compound of the present inventionto the receptors is lower than that of conventional labeled compounds,and binding of the labeled compound to the receptors and dissociation ofthe labeled compound from the receptors occurs in a relatively shorttime, and hence PET measurements can be carried out efficiently.

[0051] There are no particular limitations on the PET measurement methodin which the labeled compound of the present invention is used, with itbeing possible to carry out the PET measurements following a method thathas been publicly known from hitherto (H. Onoe, O. Inoue, K. Suzuki, H.Tsukada, T. Itoh, N. Mataga and Y. Watanabe, Brain Research 663, 191-198(1994)). Specifically, tomographic images are obtained of the brain of asubject such as a non-anesthetized rhesus monkey using a nuclearmagnetic resonance tomographic imaging (MRI) apparatus, and regions ofinterest (ROIs) are determined based on the tomographic images. Next,the labeled compound of the present invention is administered to thesubject from a vein thereof, and PET measurements are carried out on theregions of interest using a PET system. In the PET system, annihilationphotons that are emitted through the combination of positrons emittedfrom an emitting nuclear species possessed by the labeled compound andmatter-constituting electrons in the surroundings, i.e. γ-rays, aremeasured. Furthermore, if necessary the measurement data obtained can beprocessed using image reconstruction software to obtain images of theregions of interest. Here, from the standpoint of being able todistinguish fine details of the brain of the subject even in the casethat the brain is small, it is preferable to use a combination of a PETsystem having excellent spatial resolution (e.g. SHR-7700made byHamamatsu Photonics K.K.) and image reconstruction software (e.g. SHRControl II made by Hamamatsu Photonics K.K.).

EXAMPLES

[0052] Following is a more specific description of the present inventionthrough examples and comparative examples; however, the presentinvention is not limited to the following examples.

EXAMPLE 1

[0053] [¹¹C](+)N-ethyl-3-piperidyl benzilate (hereinafter referred to as‘[¹¹C ](+)3-NEPB’) was synthesized following the procedure describedbelow.

[0054] (Synthesis of ¹¹C-labeled ethyl iodide ([¹¹C]ethyl iodide)

[0055]¹¹C-labeled carbon dioxide that had been manufactured through a¹⁴N(p,α) ¹¹C reaction using a cyclotron, Sumitomo Heavy Industries,Ltd., Cypris HM-18, and methyl magnesium bromide were reacted together,and the product obtained was reduced with lithium aluminum hydride andthen treated with hydroiodic acid, thus obtaining ¹¹C-labeled ethyliodide. This synthesis was carried out using an automatic synthesisapparatus made by Sumitomo Heavy Industries, Ltd. The ¹¹C-labeled ethyliodide obtained was purified using gas chromatography (using aChromosorb W HP 80/100 column made by GL Sciences Inc.).

[0056] Synthesis of (+)3-piperidyl benzilate ((+)3-PB)

[0057] 30 ml of benzene, 0.4 g (4 mmol) of 3-piperidinol and 1.0 g (4mmol) of methyl benzilate were put into a 50 ml 3-mouth flask equippedwith a stirrer, a reflux condenser, and a molecular sieve 4A (15 g) tubeand a soda lime tube for trapping methanol from out of the solvent thatdrips down through the reflux condensation, and the mixture was refluxedby heating while stirring. After all of the methyl benzilate haddissolved, 20 mg of sodium methoxide (made by Aldrich) was added to thesolution and reaction was carried out for 3 hours. After the reactionhad been completed, the reaction liquid was cooled to room temperature,and then 50 ml of 1N hydrochloric acid was added, and the organicsolvent layer was removed. The aqueous solution layer obtained waswashed twice with 50 ml of ether, and was then made basic using a 28%ammonium hydroxide aqueous solution. The aqueous solution was subjectedto ether extraction, the ether layer obtained was washed twice with 50ml of water, and then drying was carried out using anhydrous potassiumcarbonate. After removing the drying agent, the solvent was evaporatedoff under reduced pressure, thus obtaining an oily residue, and thenthis residue was recrystallized from ether-hexane, thus obtaining 0.5 gof (±)3-piperidyl benzilate ((±)3-PB) as crystals (yield 40%).

[0058] The structure of the compound obtained was verified using ¹H-NMRand mass spectrometry.

[0059]¹H-NMR (δ, ppm): 7.36 (m, 10H), 4.92 (m, 1H), 2.94 (m, 1H), 2.70(m, 4H), 1.82 (m, 1H), 1.71 (m, 1H), 1.47 (m, 2H), 1.26 (m, 4H)

[0060] Mass spectrometry: (m/z (relative intensity)): 312 (0.16), 183(35.5), 105 (45.1), 83 (100), 77 (30.0)

[0061] The compound was subjected to optical resolution using thefollowing HPLC system (A).

[0062] HPLC system (A):

[0063] Column: Daiseru Chemicals Chiralcel OJ column (4.6×250 mm)Solvent: Hexane/ethanol mixed solvent (mixing ratio:80/20, v/v)

[0064] Flow rate: 0.5 ml/min

[0065] Synthesis of [¹¹C](+)N-ethyl-3-piperidyl benzilate([¹¹C](+)3-NEPB)

[0066] The ¹¹C-labeled ethyl iodide was trapped at −40° C. in a 2 mlreaction vial into which had been placed 0.5 mg of the (+)3-PB that hadbeen dissolved in 200 μl of DMF, this being in a helium stream (150ml/min) through P₂O₅ and a soda lime trap. Once the amount of radiationhad reached a maximum, the vial was heated at 100° C. for 3 minutes. Thevial was then cooled, and the reaction mixture was purified using thefollowing HPLC system.

[0067] HPLC system (B):

[0068] Column: MegaPak SIL C18-10 column made by JASCO Corporation(7.8×300 mm)

[0069] Solvent: Acetonitrile/30 mM ammonium acetate/acetic acid mixedsolution (400/600/2, v/v/v)

[0070] Flow rate: 6 ml/min

[0071] The solvent was removed from the fraction obtained in this way,and then the residue was dissolved in 10 ml of physiological saline, andfiltering was carried out through a 0.22 μm sterilization filter into a10 ml multi-dose vial, thus obtaining the target compound. The yield ofthe compound obtained based on the ¹¹C-labeled alkyl iodide, theradioactivity yield, the specific radioactivity and the radiochemicalpurity were measured using the HPLC system (B) described above and thefollowing HPLC system (C). HPLC system (C):

[0072] Column: FinePak SIL C18S column made by JASCO Corporation (4.6×50mm)

[0073] Solvent: Acetonitrile/ammonium acetate/acetic acid mixed solution(500/500/1, v/v/v)

[0074] Flow rate: 2 ml/min

EXAMPLE 2

[0075] [¹¹C](+)N-propyl-3-piperidyl benzilate ([¹¹C](+)3-NPPB) wassynthesized following the procedure described below.

[0076] Synthesis of ¹¹C-labeled propyl iodide ([¹¹C]propyl iodide)

[0077]¹¹C-labeled carbon dioxide that had been manufactured through a¹⁴N (p,α) ¹¹C reaction using a cyclotron, Sumitomo Heavy Industries,Ltd., Cypris HM-18, and ethyl magnesium bromide were reacted together,and the product obtained was reduced with lithium aluminum hydride andthen treated with hydroiodic acid, thus obtaining ¹¹C-labeled propyliodide. This synthesis was carried out using an automatic synthesisapparatus made by Sumitomo Heavy Industries, Ltd.

[0078] Synthesis of [¹¹C](+)N-propyl-3-piperidyl benzilate([¹¹C](+)3-NPPB)

[0079] The [¹¹C](+)N-propyl-3-piperidyl benzilate was synthesized as inExample 1, except that the ¹¹C-labeled propyl iodide obtained throughthe synthesis described above was used instead of ¹¹C-labeled ethyliodide. The yield based on the ¹¹C-labeled alkyl iodide, theradioactivity yield, the specific radioactivity and the radiochemicalpurity were measured for the compound obtained as in Example 1.

EXAMPLE 3

[0080] [¹¹C]N-ethyl-4-piperidyl benzilate ([¹¹C]4-NEPB) was synthesizedfollowing the procedure described below.

[0081] Synthesis of 4-piperidyl benzilate

[0082] 4-piperidyl benzilate was obtained as in Example 1, except that4-piperidinol was used instead of 3-piperidinol. Note, however, thatbecause optical isomers do not exist for the compound obtained, theoptical resolution of Example 1 was not carried out.

[0083] Synthesis of [¹¹C]N-ethyl-4-piperidyl benzilate

[0084] The [¹¹C]N-ethyl-4-piperidyl benzilate was synthesized as inExample 1, except that the 4-piperidyl benzilate obtained through thesynthesis described above was used instead of (+)3-piperidyl benzilate.The yield based on the ¹¹C-labeled alkyl iodide, the radioactivityyield, the specific radioactivity and the radiochemical purity weremeasured for the compound obtained as in Example 1.

EXAMPLE 4

[0085] Synthesis of [¹¹C]N-propyl-4-piperidyl benzilate

[0086] The [¹¹C]N-propyl-4-piperidyl benzilate was synthesized as inExample 1, except that the ¹¹C-labeled propyl iodide obtained in Example2 was used instead of ¹¹C-labeled ethyl iodide, and the 4-piperidylbenzilate obtained in Example 3 was used instead of (+)3-piperidylbenzilate. The yield based on the ¹¹C-labeled alkyl iodide, theradioactivity yield, the specific radioactivity and the radiochemicalpurity were measured for the compound obtained as in Example 1.

COMPARATIVE EXAMPLE 1

[0087] Synthesis of [¹¹C](+)N-methyl-3-piperidyl benzilate

[0088] [¹¹C](+)N-methyl-3-piperidyl benzilate was synthesized as inExample 1, except that ¹¹C-labeled methyl iodide was used instead of¹¹C-labeled ethyl iodide. The yield based on the ¹¹C-labeled alkyliodide, the radioactivity yield, the specific radioactivity and theradiochemical purity were measured for the compound obtained as inExample 1, except that an acetonitrile/0.1M ammonium acetate/acetic acidmixed solution (500/500/5, v/v/v) was used as the solvent in the HPLCsystem.

COMPARATIVE EXAMPLE 2

[0089] Synthesis of [¹¹C]N-methyl-4-piperidyl benzilate

[0090] [¹¹C]N-methyl-4-piperidyl benzilate was synthesized as in Example1, except that ¹¹C-labeled methyl iodide was used instead of ¹¹C-labeledethyl iodide, and 4-piperidyl benzilate was used instead of(+)3-piperidyl benzilate. The yield based on the ¹¹C-labeled alkyliodide, the radioactivity yield, the specific radioactivity and theradiochemical purity were measured for the compound obtained as inExample 1.

[0091] For each of the compounds of Examples 1 to 4 and ComparativeExamples 1 and 2 obtained as described above, the retention time in theseparating column of HPLC system (B), the retention time in theanalyzing column of HPLC system (C), the yield based on the ¹¹C-labeledalkyl iodide, the radioactivity yield, the specific radioactivity andthe radiochemical purity are shown in Table 1. Moreover, the results ofthe HPLC analysis on the ¹¹C-labeled alkyl iodides and final productsobtained in the examples and comparative examples are shown in FIGS. 2Ato 2E, FIGS. 3A to 3C and FIGS. 4A to 4C.

[0092]FIGS. 2A to 2E are graphs showing the HPLC analysis results forthe ¹¹C-labeled alkyl iodides; FIG. 2A was obtained from measurements onthe ¹¹C-labeled ethyl iodide before separation by gas chromatography,FIG. 2B from measurements on the ¹¹C-labeled ethyl iodide afterseparation by gas chromatography, FIG. 2C from measurements on the¹¹C-labeled propyl iodide before separation by gas chromatography, FIG.2D from measurements on the ¹¹C-labeled propyl iodide after separationby gas chromatography, and FIG. 2E from measurements on ¹¹C-labeledmethyl iodide.

[0093]FIGS. 3A to 3C are graphs showing the HPLC analysis results forthe final products obtained in Examples 1 and 2 and Comparative Example1; FIG. 3A was obtained from measurements on [¹¹C](±)3-NMPB, FIG. 3Bfrom measurements on [¹¹C](±)3-NEPB, and FIG. 3C from measurements on[¹¹C](±)3-NPPB.

[0094]FIGS. 4A to 4C are graphs showing the HPLC analysis results forthe final products obtained in Examples 3 and 4 and Comparative Example2; FIG. 4A was obtained from measurements on [¹¹C]4-NMPB, FIG. 4B frommeasurements on [¹¹C]4-NEPB, and FIG. 4C from measurements on[¹¹C]4-NPPB. TABLE 1 Retention time (mm) Radioactivity SpecificRadiochemical Separating Analyzing Yield yield radioactivity purityCompound column column (%) (MBq) (GBq/mol) (%) Example 1 [¹¹C](+)3-NEPB12.8 6.5 38 309 66.3 >99 Example 2 [¹¹C](+)3-NPPB 14.6 8.9 32 39534.1 >99 Example 3 [¹¹C]4-NEPB 9.9 5.0 42 336 337 >99 Example 4[¹¹C]4-NPPB 11.1 7.0 35 337 52.6 >99 Comparative [¹¹C](+)3-NMPB (7.1)5.4 70 2710 67.4 >99 Example 1 Comparative [¹¹C]4-NMPB (5.5) 4.1 59 163048.0 >99 Example 2

[0095] Next, PET measurements were carried out using the compoundsobtained in Examples 1 to 4 and Comparative Examples 1 and 2 followingthe method described in the undermentioned document:

[0096] H. Onoe, O. Inoue, K. Suzuki, H. Tsukada, T. Itoh, N. Mataga andY. Watanabe, Brain Research 663, 191-198 (1994)

[0097] First, a subject, a non-anesthetized rhesus monkey of body weightapproximately 5 kg, was fastened to the PET apparatus (SHR-7700 made byHamamatsu Photonics K.K.), and the transmission was measured forabsorptive correction of the PET measurements. Approximately 200 MBq ofthe compound of one of Examples 1 to 4 and Comparative Examples 1 and 2was then administered to the subject from a vein thereof, and dynamicmeasurements were carried out for 91 minutes. Moreover, for each set ofmeasurements, measurements were carried out in advance on the subjectusing a nuclear magnetic resonance tomographic imaging (MRI) apparatusto obtain tomographic images, and the positions of the cerebellum, thehippocampus, the occipital lobe, the basal ganglia (striatum), thetemporal lobe, the frontal lobe and the cingulate gyrus were determinedbased on the tomographic images; changes in the amount of γ-rays overtime were measured for each of these regions of interest. The resultsare shown in FIGS. 5A to 5C and FIGS. 6A to 6C.

[0098]FIGS. 5A to 5C are graphs showing the relationship between timeand radioactivity concentration for each of the regions of interest,with FIG. 5A being for the case that [¹¹C](+)3-NMPB was used, FIG. 5Bbeing for the case that [¹¹C](+)3-NEPB was used, and FIG. 5C being forthe case that [¹¹C](+)3-NPPB was used.

[0099]FIGS. 6A to 6C are graphs showing the relationship between timeand radioactivity concentration for each of the regions of interest,with FIG. 6A being for the case that [¹¹C]4-NMPB was used, FIG. 6B beingfor the case that [¹¹C]4-NEPB was used, and FIG. 6C being for the casethat [¹¹C]4-NPPB was used.

[0100] As shown in FIGS. 5B, 5C, 6B and 6C, in the case that a compoundof the present invention was used, the results obtained were that theradioactivity concentration corresponding to the distribution of thelabeled compound in the basal ganglia and the occipital lobe was high,and moreover the radioactivity concentration was also relatively highfor the frontal lobe and the temporal lobe. These results agreeextremely well with the distribution of acetylcholine nervous systemmuscarinic receptors that has been known from hitherto, and hence it wasverified that the compounds of the present invention are able to bindspecifically to these receptors. Note that the reason that the absolutevalue of the uptake of the compounds of the present invention into thebrain is low compared with the compounds of the comparative examples isthat the compounds of the present invention have a low liposolubility.Carrying out a comparison using the value of the uptake of the labeledcompound in each region of interest relative to the uptake in thecerebellum, higher values were obtained in the case that the compoundsof the present invention were used.

[0101] Moreover, as is clear from comparing FIGS. 5B, 5C, 6B and 6C,which show the results for the cases that compounds of the presentinvention were used, with FIGS. 5A and 6A, which show the results forthe cases that conventional labeled compounds were used, it was verifiedthat the compounds of the present invention have a lower affinity to thebrain, and binding and dissociation between the compounds of the presentinvention and the receptors occur in a relatively short time.

[0102] Next, PET measurements were carried out using the [¹¹C](+)3-NPPBobtained in Example 2 and the [¹¹C]4-NMPB obtained in ComparativeExample 2 using the method described above, except that an intrinsicacetylcholine decomposition enzyme inhibitor E2020 (Aricept) wasadministered to the subject in advance. Upon being administered to asubject, E2020 has a function of increasing the amount of acetylcholinein the synaptic clefts and increasing the blood flow amount in thebrain; by carrying out PET measurements using this drug, the dependenceof the labeled compound on the blood flow amount in the brain can beevaluated.

[0103]FIGS. 7A to 7C are graphs showing the relationship betweenmeasurement time and radioactivity concentration obtained from PETmeasurements using the [¹¹C](+)3-NPPB; FIG. 7A shows the case that E2020was not administered, FIG. 7B the case that 50 μg/kg of E2020 wasadministered, and FIG. 7C the case that 250 μg/kg of E2020 wasadministered.

[0104] Moreover, FIGS. 8A to 8C are graphs showing the relationshipbetween measurement time and radioactivity concentration for each of theregions of interest obtained from PET measurements using the[¹¹C]4-NMPB; FIG. 8A shows the case that E2020 was not administered,FIG. 8B the case that 50 μg/kg of E2020 was administered, and FIG. 8Cthe case that 250 μg/kg of E2020 was administered.

[0105] In the case that the [¹¹C]4-NMPB was used, it was found thatthere was an increase in the uptake of the labeled compound into thebrain upon an increase in the blood flow amount. On the other hand, inthe case that the [¹¹C]4-NPPB was used, influence due to an increase inthe blood flow amount in the brain was not found. Moreover, in the casethat the [¹¹C]4-NPPB was used, it was found that the rate ofdissociation of the [¹¹C]4-NPPB from the receptors increased upon anincrease in the amount of E2020 administered, and that competition inbinding to the receptors occurred between intrinsic acetylcholine andthe [¹¹C]4-NPPB.

INDUSTRIAL APPLICABILITY

[0106] As described above, according to the present invention it ispossible to obtain a muscarinic acetylcholine nervous system labeledcompound that enables PET measurements to be carried out efficientlywith improved precision and without there being influence from changesin the blood flow amount in the region of interest.

1. A muscarinic acetylcholine nervous system labeled compound forpositron emission tomography measurement, characterized by having astructure represented by the following general formula (I):

[in the formula, W represents one selected from the group consisting ofgroups represented by the following formulae (II) and (III):

(in the formulae, R represents one selected from the group consisting ofan ¹¹C-labeled ethyl group and an ¹¹C-labeled propyl group), and in thecase that W is a group represented by the formula (II), the formula (I)is the (+)-isomer].
 2. A method of manufacturing a muscarinicacetylcholine nervous system labeled compound for positron emissiontomography measurement, characterized by comprising a step of obtaininga compound represented by the following formula (I):

from an ¹¹C-labeled alkyl halide represented by the following formula(IV): R—X   (IV) and a benzilic acid piperidyl ester represented by thefollowing formula (V):

[In the formulae, R represents one selected from the group consisting ofan ¹¹C-labeled ethyl group and an ¹¹C-labeled propyl group, X representsa halogen atom, W′ represents one selected from the group consisting ofa (+)3-piperidyl group and a 4-piperidyl group, and W represents oneselected from the group consisting of groups represented by thefollowing formulae (II) and (III):

(in the formulae, R represents one selected from the group consisting ofan ¹¹C-labeled ethyl group and an ¹¹C-labeled propyl group).]
 3. Apositron emission tomography measurement method, comprising the stepsof: administering the labeled compound according to claim 1 to asubject, thus labeling muscarinic acetylcholine nervous system receptorsof said subject with said labeled compound; and measuring γ-rays emittedthrough the combination of positrons emitted from an emitting nuclearspecies possessed by said labeled compound and matter-constitutingelectrons.